handle the situatioin when giving a NaN prod target to injectors

When all production wells are under individual controls, the group can
have zero production guide rates, which can result NaN value targets for
injectors.

opm/core/wells/WellsGroup.cpp

@@ -1075,7 +1075,7 @@ namespace Opm
                                         const bool only_group)
     {
         if ( !isInjector() ) {
-            // assert(target == 0.0);
+            assert(target == 0.0 || std::isnan(target));
             return;
         }
 
@@ -1280,7 +1280,7 @@ namespace Opm
                                          const bool only_group)
     {
         if ( !isProducer() ) {
-            assert(target == 0.0);
+            assert(target == 0.0 || std::isnan(target));
             return;
         }